Method for producing manganese-zinc ferrite

ABSTRACT

MANGANESE-ZINC FERRITE FOR A MAGNETIC HEAT WHICH HAS A HIGH INITIAL PERMEABILITY AND CAUSES FEW FERRITE NOISE AND IMPROVES THE RESISTANCE TO THE DROPPING OUT OF FERRITE CRYSTALLITES GENERATED BY THE RUBBING OF A MAGNETIC TAPE, IS PRODUCED BY ADDING 0.001-0.06% BY WEIGHT OF AT LEAST ONE OF ALKALI METAL OXIDES SELECTED FROM THE GROUP CONSISTING OF SODIUM OXIDE AND POTASSIUM OXIDE, 0.0050.04% BY WEIGHT OF RARE EARTH METAL OXIDES AND 0.63.0% BY WEIGHT OF INDIUM OXIDE TO THE STARTING MATERIALS OF FERRITE CONSISTING MAINLY OF FERRIC OXIDE, MANGANESE OXIDE AND ZINC OXIDE, MOLDING THE RESULTING MIXTURE, FIRING THE MOLDED BODY AT A TEMPERATURE OF 1,100-1,300*C. UNDER ATMOSPHERE OF A REDUCED PRESSURE OF LOWER THAN 10-2 MM. HG, FIRING THE THUS TREATED MOLDED BODY AT A TEMPERATURE OF 1,250-1,500*C. UNDER A HELIUM ATMOSPHERE CONTAINING 0.5-20% BY VOLUME OF OXYGEN AND COOL ING THE SINTERED BODY.

United States Patent 3,836,472 METHOD FOR PRODUCING MANGANESE-ZINCFERRITE Shigeo Soejima, Nagoya, and Hideo Irokawa, Chiryu, gapan,assignors to NGK Insulators, Ltd., Nagoya,

apan No Drawing. Filed Dec. 4, 1972, Ser. No. 311,621 Claims priority,application Japan, Dec. 14, 1971, 46/100,626; Sept. 21, 1972, 47/94,116Int. Cl. C0411 35/38, 35/40 US. Cl. 252-62.57 3 Claims ABSTRACT OF THEDISCLOSURE Manganese-zinc ferrite for a magnetic head which has a highinitial permeability and causes few ferrite noise and improves theresistance to the dropping out of ferrite crystallites generated by therubbing of a magnetic tape, is produced by adding 0.0010.06% by weightof at least one of alkali metal oxides selected from the groupconsisting of sodium oxide and potassium oxide, 0.005- 0.04% by weightof rare earth metal oxides and 0.6 3.0% by weight of indium oxide to thestarting materials of ferrite consisting mainly of ferric oxide,manganese oxide and zinc oxide, molding the resulting mixture, firingthe molded body at a temperature of 1,100-1,300 C. under atmosphere of areduced pressure of lower than mm. Hg, firing the thus treated moldedbody at a temperature of 1,250l,500 C. under a helium atmospherecontaining 0.5% by volume of oxygen and cooling the sintered body.

BACKGROUND OF THE INVENTION Field of the Invention The present inventionrelates to a method for producing ferrite and particularly to a methodfor producing manganese-zinc ferrite having a high initial permeabilityin which the dropping out of ferrite crystallites caused by rubbing of amagnetic tape can be prevented when said manganese-zinc ferrite is usedfor a magnetic head.

Description of the Prior Art Recently, the use of ferrites has beenincreased instead of the previous Permalloy and other metallic materialsas a material for magnetic head, because ferrites have a high hardnessand a high abrasion resistance against the rubbing of a magnetic tapeand have excellent high frequency properties.

In general, the ferrite for a magnetic head needs that the initialpermeability and the magnetic flux density are high and it is veryimportant that the ferrite crystallites in the vicinity of the gap atthe track surface are not dropped out due to the rubbing of the magnetictape. Because if even a part of the ferrite crystallites of the magnetichead at the track surface is dropped out, the ferrite crystallites aredropped out successively and the recording or reproducing efiiciency ofthe magnetic head is not only decreased but also the life isconsiderably reduced and further the magnetic tape is seriously damaged.

The previous method for producing the ferrite for magnetic head includesmonocrystallizing process, hot press process and vacuum sinteringprocess.

The ferrite produced by the monocrystallizing process is considerablyinfluenced in the properties by the production factors and therefore itis difi'icult to maintain the quality constantly and further saidferrite thus produced has an orientation in the properties andaccordingly it is necessary to cut the ferrite so as to agree accuratelywith this orientation and this process is very technically troublesome.

Patented Sept. 17, 1974 The hot press process is a very complicatedprocess and is not suitable for the mass-production, because thisprocess comprises inserting the shaped body into a mold, compressing theshaped body at a high temperature and taking out the ferrite from thestate where the mold, powdery alumina of a compression medium andferrite are tightly sticked.

Furthermore, Yuzo Shichijo et al. have proposed a vacuum sinteringprocess (Journal of Applied Physics vol. 35, pages 1,646l,647, 1964)wherein the primary firing stage is effected under an atmosphere of areduced pressure and the succeeding firing stage is performed undernitrogen atmosphere containing a small amount of oxygen.

In this process, since the primary firing stage is effected under anatmosphere of a reduced pressure, the gas enclosed in the space amongthe particles of the shaped body, which retards elimination of pores inthe sintering, is removed and further vacancies of oxygen are formed ina high concentration in the crystal structure of ferrite and masstransfer is made easy and the elimination of pores in the sintering ispromoted. Furthermore, in order to obtain a high permeability, asecondary firing (succeeding firing stage) is performed under nitrogenatmosphere containing a small amount of oxygen to effect a moderateoxidation. However, since the atmosphere for the secondary firing isnitrogen containing a small amount of oxygen, nitrogen also enters intocrystal grains as the oxidation proceeds but the diffusion constant ofnitrogen is low, so that nitrogen remains in the interior of thesintered body and accumulates with the progress of sintering to formpores, which lower the density and permeability.

In addition, British Pat. No. 1,071,611 discloses a method for producingferrite wherein the above described 'vacuum sintering process is adoptedand further additives, such as In O CaO and the like, are added to thestarting materials of ferrite. But in this method it is difficult tomake the pores remained in ferrite less than 0.5% and the remainingpores cause the dropping out of ferrite crystallites at the tracksurface owing to the rubbing of the magnetic tape.

The sintered ferrite produced by the above described vacuum sinteringprocess is low in the strength of the grain boundary between the ferritecrystallites and therefore it is impossible in this process to preventthe dropping out of ferrite crystallites. In this process, it isconsidered to enlarge the crystallite size in order to decrease thedropping out of ferrite crystallites, but the specific noise of ferrite(ferrite noise) which is generated in the use of a magnetic head, ismainly based on a large size of ferrite crystallites and therefore themeans for preventing the dropping out by enlarging the crystallite sizeis not adequate.

Ferrites obtained by conventional methods for producing sintered ferritehave micro-structures which are uneven in the crystallite size, so thatthey cause the dropping out and ferrite noise and are not suitable forferrite for a magnetic head.

SUMMARY OF THE INVENTION The object of the present invention is toprovide a method for producing manganese-zinc ferrite for a magnetichead which can prevent the dropping out of ferrite crystallitesgenerated by the rubbing of a magnetic tape and has an improved initialpermeability and cause few ferrite noise.

The foregoing object and other objects as well as the characteristicfeatures of the present invention will become more apparent and morereadily understandable by the following description and the appendedclaims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A preferred embodimentfor producing the ferrite of the present invention will be explained indetail.

The method for producing the ferrite of the present invention comprisesadding 0.001-0.06% by weight of at least one of alkali metal oxidesselected from the group consisting of Na O and K 0, 0.005-0.04% byweight of at least one of rare earth metal oxides selected from thegroup consisting of Y O La o (3e0 Pr O Nd O and Sm O and 06-30% byweight of Im O to the starting materials for manganese-zinc ferriteconsisting mainly of 49-55 mol percent of F2 35-15 mol percent of MnOand 30-10 mol percent of ZnO, preferably 50-53 mol percent of Fe O 30-20mol percent of M110 and 27-17 mol percent of ZnO, mixing the resultingmaterials in a steel ball mill by a wet process, calcining the mixtureat a temperature of 800-1,100 C., crushing the calcined mixture in asteel ball mill by a wet process to obtain the calcined powder having agrain size of 1-2,u., molding the thus obtained calcined powder addedwith a binder, such as distilled water by a conventional moldingprocess, for example press molding or rubber press molding, firing themolded body in a firing furnace at a temperature of 1,100-l,300 C. for1-20 hours under an atmosphere of a reduced pressure of lower than 10mm. Hg and then firing the sintered body at a temperature of 1,250-1,500C. for 2-8 hours under a helium atmosphere containing -20% by volume ofoxygen and cooling the thus sintered body under an atmosphere containinga small amount of oxygen, such as a nitrogen atmosphere.

Furthermore, the above described alkali metal oxides, rare earth metaloxides and In O may be used in the form of compounds which can beconverted into these oxides through firing.

In the present invention, it has been found from a large number ofexperimental results that by the coexistence of the above describedalkali metal oxide, rare earth metal oxide and E1 0 it is possible tomake ferrite crystallites to uniform size of 20-40 and further thebonding strength between ferrite crystallites can be increased.Moreover, by the primary firing under an atmosphere of a reducedpressure of less than mm. Hg and the secondary firing under heliumatmosphere containing 05-20% by volume of oxygen, a very compact ferritehaving a porosity of less than 0.1% can be obtained. The co-efr'ect ofsuch auxiliary additive components and the firing conditions can onlyprovide the ferrite which has a high initial permeability and has nohindrance of ferrite noise, and further does not cause the dropping outof ferrite crystallites due to the rubbing of a magnetic tape.

The reason why the amounts of the above described oxides are limited asmentioned above, will be explained hereinafter.

When the amount of the alkali metal oxides is less than 0.001% byweight, the distribution of the crystallite size becomes uneven.

When the amount of the rare earth metal oxides is less tallites is notsatisfactorily improved and the prevention of the dropping out offerrite crystallites cannot be attained.

When the amount of 111 0 is less than 0.6% by weight, the residual poresdo not decrease to less than 0.1%.

On the other hand, when the amounts of the alkali metal oxide, the rareearth metal oxide and ln O exceed 0.06% by weight, 0.04% by weight and3.0% by weight, respectively, the initial permeability is noticeablydecreased.

The limitations of the primary firing temperature and the secondaryfiring temperature are based on the following reason. When thetemperatures of the primary firing and the secondary firing are lowerthan l,100 C. and 1,250 C., respectively, the residual pores cannot bedecreased to less than 0.1%, While when the temperatures of the primaryfiring and the secondary firing are higher than 1,300 C. and 1,500 C.,respectively, the volatilization of zinc is vigorous and the initialpermeability is considerably decreased.

The limitations of the atmosphere of the primary firing and thesecondary firing are based on the following reason.

When the pressure of the atmosphere of the primary firing exceeds 10-mm. Hg, it is impossible to decrease the residual porosity to less than0.1%.

When the oxygen content in the helium atmosphere is less than 0.5% byvolume or more than 20% by volume, the initial permeability isconsiderably decreased.

The following examples are given for the purpose of illustration of thisinvention and are not intended as limitations thereof.

Example 1 A main component composed of 51.5 mol percent of Fe O 23.5 molpercent of MnO and 25.0 mol percent of ZnO was added with additives inthe amounts as shown in the following Table 1.

The resulting mixture was mixed in a wet state for 20 hours in a steelball mill, calcined at 950 C. for 2 hours, crushed in a steel ball millfor 20 hours and dried. After added with distilled water as a binder,the mass was molded under a press compression of 3.5 ton/cm. to obtain agreen compact. The green compact was fired at 1,150 C. for 6 hours underan atmosphere of a reduced pressure of 10" mm. Hg further fired at1,420" C. for 2 hours under a helium atmosphere containing 12% by volumeof oxygen, and cooled under a nitrogen atmosphere to obtain ferrite No.1, No. 2, No. 3, No. 4 or No. 5.

As a control, the same main component as described above was added with1.0% by weight of In O as an additive, and the resulting mixture wasmixed, calcined, crushed and molded under the same conditions as described above to obtain a green compact. The green compact was fired at1,150 C. for 6 hours under an atmosphere of a reduced pressure of 10-mm. Hg, further fired at 1,420 C. for 2 hours under a nitrogenatmosphere containing 12% by volume of oxygen, and then cooled under anitrogen atmosphere to obtain control ferrite No. 1.

Magnetic heads were prepared from the above obtained SIX ferrites.Properties of the ferrites and properties of the than 0.005% by weight,the bond between the ferrite crysresulting magnetic heads are shown inTable 1.

TABLE 1 Time until reprodueing out ut is Additive (percent by weight)dfezgeased 10% Initial di i; m r

0 e initia value Ferrite erm Ferrite number N 1120 YzOa 111202 P z amist! fi 2 3 ggiisiiltl Preslent invention:

0. 003 0. 01 +1. 0 Longer than 1 000 Small. 1 0. 005 +0. 01 +1. 0 .dnfin 12130 8 228 33 dd 0. 01 +0. 01 +1. 0 do 16, 600 3: 485 Do 0. 03 +0.01 +1. 0 +0. 01 .do --d0 17, 400 3, 480 90 Do:

NdzOa 0. +0. 01 +1. 0 +0 01 ...d0.....-'::- ..-d0 15, 800 3, 480 90 Do.

1. 0 0. Large. 14, 500 3, 480 90 0.5.

The magnetic heads after running test were observed by a microscope. Inthe magnetic head prepared from control ferrite No. 1, largecrystallites of about 2000 and small crystallites of about 20 1. wereirregularly distributed, and a large number of ferrite crystallites weredropped of more than 4 times the life of conventional magnetic heads.

Example 3 A main component composed of 52.0 mol percent of 5 Fe O 28.0mol percent of MnO and 20.0 mol percent out near the gap due to rubbmgof emagnem tape and of ZnO was added with additives in the amounts asshown the gap was broken and the reproduemg Output lowered in thefollowing Table 3, and the resulting mixture was Furthermore, ferritenoise was large due to the presence mixed in a Wet state for 20 hours ina steel ball n of large erystalhtes f the calcined at 1,050 C. for 2hours, crushed in a wet state Whfle 111 the magnetlc heafi prelared mfemtcs N 10 for 20 hours and dried. After added with by weight 1 to 5 ofthe Present mventlon femte erystalhtes of distilled water as a binder,the mass was molded into had a uniform crystallite size of 2040,u., noferrite crystala predetermined Shape under a pressure of 35 ton/cm;lites were dropped out, the reproduclng output d1d not to obtain a greencompact The green compact was fired lower, and the life, which isrepresented by the time until at for 4 hours under an atmosphere of athe reproducing output is decreased by of the iniduced Pressure of -4 mmHg, further fired at c {ital value e more than two tunes the hfe ofeonven for 4 hours under a helium atmosphere containing 7.0% tlonalmagnetlc headsby volume of oxygen, and cooled under a nitrogen atmos-Example 2 phere to obtain ferrite No. 10. As a control, the same maincomponent as described A mam component composed of mol Percent of 0above was added with 2.5% by weight of In O and the Fe203 mol pelicentand mol percent resulting mixture was mixed, calcined, crushed andmolded P 2110 was afided wlth addltwes m the e e as shown under the sameconditions as described above. The result- 111 the following Table 2,and the resulting mixture was ing green compact was fired at 1 for 4hours mixed, calcined, crushed and molded in the same manner under anatmosphere of a reduced ressure of 104 as described i Example 1 toObtain a green g 25 Hg, further fired at 1,380 C. for 4 hours under anitrogen The resultmg green compact was fired at 1,280 atmospherecontaining 7.0% by volume of oxygen, and for 2 hours under an atmosphereof a reduced Pressure cooled under a nitrogen atmosphere to obtaincontrol of 10'' mm. Hg, further fired at 1, 320 C. f r 6 hours ferriteNo. 3. Magnetic heads were prepared from the under a helium atmospherecontaimng 1.5% by Volume above obtained two ferrites. Properties of theferrites and of y and Cooled under a nitrogen atmosphfire properties ofthe resulting magnetic heads are shown in obtain ferrite No. 6, No. 7,No. 8 or No. 9. T bl 3,

TABLE 3 Additive (p t by weight) g g i liglisi Initial iiiii digit?Ferrite numb r Naio K20 no. 090. 111.0. iii iiii ti i'iiiiii 'tfii) i5%??? @2055 $33 $523.?) Present invention, 10... 0.005 +0.005 +0005+0.02 +2.5 Longer than 1,000 Smal1 11,700 4, 250 120 Less than 0.1Control,3 .5 3 0 do 10,300 4, 250 120 0.6.

As a control, the same main component as described above was added with2.0% by weight of In O and the resulting mixture was mixed, calcined,crushed and molded under the same conditions as described above toobtain a green compact. The green compact was fired at 1,280 C. for 2hours under an atmosphere of a reduced pressure of 10* mm. Hg, furtherfired at 1,320 C. for 6 hours under a nitrogen atmosphere containing1.5% by volume of oxygen, and cooled under a nitrogen atmosphere toobtain control ferrite No. 2.

Magnetic heads were prepared from the five ferrites obtained above.Properties of the ferrites and properties of the resulting magneticheads are shown in Table 2.

Microscopic observation of the magnetic heads after running test showedthat although both of the ferrite No. 10 of the present invention andthe control ferrite No. 3 were composed of crystallites of 2040/L, inthe magnetic head prepared from control ferrite No. 3, ferritecrystallites were dropped out, the gap was broken and the reproducingoutput lowered, while in the magnetic head prepared from ferrite No. 10of the present invention, no ferrite crystallites were dropped out, andthe reproducing output did not lower.

As described above, according to the present invention, ferrites havinga high initial permeability and composed of crystallites having auniform size of 20-40 TABLE 2 Time until reproducing output is MagneticCurie Additive (percent by weight) decreased by 10% Initial fluxtemperof the initial value Ferrite permedensity ature Porosity Ferritenumber K20 LazOd 111203 Plzos (hr.) noise ability (gauss) 0.) (percent)Present invention:

6 0, 003 +0. 015 +2.0 Longer than 1,000 Sma1l. 15, 800 3, 480 90 Lessthan 0.1.

0. 005 +0. 015 +2. 0 d0 d0 16, 200 3, 475 90 Do. 0. 01 +0. 015 +2.0 +0.015 ..do do 16, 200 3, 475 90 Do.

SmzOa 0.02 +0.015 +2.0 +0 015 ...d0-... ...do 15,900 3,480 90 Do.Control, 2 2. U Large. 15, 200 3, 480 90 0.6.

Microscopic observation of the magnetic heads after running test showedthat in the magnetic head prepared from control ferrite No. 2, althoughthe ferrite was composed of relatively fine crystallites of 1020,u, alarge amount of ferrite crystallites were dropped out near the gap, andthe gap was broken, while in the magnetic head prepared from ferritesNo. 6 to No. 9 of the present invention, no ferrite crystallites weredropped out, and the gap kept the initial state.

As seen from Table 2, the magnetic head prepared from the ferriteaccording to the present invention has a life can be obtained by aco-eifect of the additives and the firing conditions, and accordinglygeneration of ferrite noise in the magnetic head prepared from theferrite is small, and further since the ferrite has a low porosity and ahigh bonding strength between the crystallites, dropping out of ferritecrystallites due to rubbing of magnetic tape hardly occurs in themagnetic head. Therefore, when the ferrite is used as a magnetic head,the head has a life of more than 2 to 4 times the life of conventionalmagnetic heads, and the ferrite of the present invention is considerablyuseful in industry.

7 What is claimed is: 1. A method for producing manganese-zinc ferritewhich comprises adding 0.001-0.06% by weight of at least one of alkalimetal oxides selected from the group consisting of sodium oxide andpotassium oxide, ODDS-0.04% by weight of at least one of rare earthmetal oxides selected from the group consisting of Y O La O C302, PI203,Ndzog and 8111203, and weight of indium oxide to starting materials offerrite consisting of 49-55 mol percent of ferric oxide, 35-15 molpercent of manganese oxide and 30-10 mol percent of zinc oxide, moldingthe resulting mixture, firing the molded body at a temperature of 1,100-

1,300 C. under an atmosphere of a reduced pressure of lower than 10' mm.Hg, again firing the thus treated molded body at a temperature of1,2501500 C. under a helium atmosphere containing 05-20% by volume ofoxygen and cooling the sintered body under a nitrogen atmosphere. 2. Amethod as claimed in claim 1, wherein said rare earth metal oxide is atleast one of the oxides selected from the group consisting of Y O L21 Oand CeO 3. A method as claimed in claim 1, wherein the startingmaterials of ferrite are 50-53 mol percent of ferric oxide, 30-20 molpercent of manganese oxide and 27-17 mol percent of zinc oxide.

References Cited UNITED STATES PATENTS 2/1960 Sasaki et al. 252-62.629/1970 Akashi et al. 252-62.62X

FOREIGN PATENTS 1,071,611 6/1967 Great Britain.

OTHER REFERENCES Shichijo et al. Journal of Applied Physics, vol. 35,pp. 1646-7, 1964.

20 JACK COOPER, Primary Examiner US. Cl. X.R.

